In certain situations and circumstances, it is important to know if the insulation surrounding a wire or cable has deteriorated. Such deterioration could be a precursor to a failure in an important system. For example, in jet fighter aircraft, wire chaffing and the resulting deterioration of the insulation are often precursors to failure. If the chaffing continues undetected, the cable may be severed or shorted to another cable with catastrophic results depending on the purpose of the cable. If the cable is used to control the rudder or aileron, severing or shorting of that cable could result in a loss of control of the aircraft and possibly a fatal crash. It would be desirable therefore to be able to detect deterioration of wire insulation before failure occurs. If insulation defects and deterioration are detected in a timely manner, the wire or cable could be replaced before a catastrophic failure occurs.
U.S. Pat. No. 3,096,478 discloses an apparatus for detecting non-uniformity in electrically insulated wires through the use of conductive gas electrodes. The electrodes consist of a tube or sleeve containing ionized air which establishes a direct current path through the defective insulation segment. In the apparatus disclosed in this patent, the cable must be placed inside the conductive gas electrode. The conductor of the cable itself must be grounded and the cable must be moved through the conductive gas electrode during the test, so that the system can be employed essentially only for testing cables prior to installation. A current or voltage of fixed magnitude as applied to the, or one, electrode and a fault indication is based on a change in current flow through the cable insulation.
U.S. Pat. No. 3,263,165 describes apparatus for detecting corona-producing cable insulation defects. For this purpose, a voltage gradient is established along the cable by electrodes containing an ionized gas, the voltage being sufficient to cause gaseous areas in the cable insulation to become ionized and produce high frequency radiation which is detected. The cable is advanced through the apparatus and does not itself conduct any signal. Here again, the cable must be tested prior to installation.
The systems disclosed in both of these references require the measurement of very small currents in the cable under test and require that the cable not be in an installed state. These requirements considerably limit the range of applications of such systems.
U.S. Pat. No. 3,639,831 discloses a method and apparatus for producing a directable, electrically conducting gas jet and detecting the presence of anomalies therein caused by insulators, conductors or semiconductors. The gas jet flows across a test zone and impinges upon a target anode which is maintained at a bias potential with respect to the cathode of the ionizing generator such as to cause an electrical current to flow between the anode and the cathode via the gas jet across the test zone. This device requires that the electrical current flow between the gas jet nozzle and the target anode be constant. Moreover, there is no provision for measuring anomalies which have their own potential or are carrying a current.
Allowed U.S. patent application Ser. No. 07/267,138, filed on Nov. 4, 1988, discloses devices for detecting cable insulation defects which utilize an ionized gas stream or cloud in a manner which differs fundamentally from the techniques described in the above-cited patents. These devices can be used to test installed cables which are connected in an electrical system and are carrying their normal signals.
The devices disclosed in U.S. patent application Ser. No. 07/267,138 include an ion source and at least one sense conductor, both disposed adjacent the cable to be tested while the latter is carrying a normal alternating voltage. According to one embodiment, the ion source is moved along the cable under test to produce a localized ion cloud forming a conductive region between the cable and a single sense conductor. When a cable insulation defect of sufficient severity is encountered by the ion cloud, the electric field produced by the signal on the cable conductor induces a corresponding signal on the sense conductor. This occurrence can be detected by performing synchronous detection of the sense wire signal with respect to the cable conductor signal. The location of an insulation defect can be determined by noting the position of the ion source at the moment when a synchronous detection output signal is being produced.
According to a second embodiment disclosed in the above-cited application, use is made of an extended ion source extending along a length of the cable under test to generate an ion cloud which is coextensive with the source, and three position sense conductors are disposed along the cable length and are coded with insulation segments which form, for example, a gray Code such that along each portion of the cable length the position sense conductors present a unique combination of insulated segments and bare wire. A cable insulation defect will induce a current in phase with the cable signal on those sense conductors which have bare portions opposite the location of the defect and the signal on each sense conductor can be synchronously detected with respect to the cable signal.
A major disadvantage of this technique is that each coded sense conductor will receive a relatively low current level signal which can be sensed only with difficulty, particularly when testing is carried out in the presence of higher level noise signals.
It has also been proposed to provide an extended ion source which produces a localized ion cloud that travels along the cable. This is achieved by dividing the source into several segments which are actuated in sequence. For example, individual ion sources can be activated in sequence by multi-phase ionizing burst conductors each connected to supply ionizing energy to a respective source. The number of identifiable locations for the localized ion cloud is equal to the number of source segments or burst conductor phases. Such ion sources are disclosed in pending U.S. patent application Ser. No. 267,150, filed on Nov. 4, 1988.
According to the disclosure of that application, the current in the cable under test is monitored to determine whether, when the ion source is activated, a current component appears in the cable conductor as a result of an insulation defect. The current component thus induced in the cable conductor will be of small magnitude and hence difficult to detect. In addition, the conductor under test must be disconnected from its associated circuit devices since a signal in the conductor would effectively mask the current resulting from an insulation defect.